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The chemical bonding topology in the MgB2 superconductor is similar to that in the high Tc cuprates. Thus the conducting skeleton of MgB2 consists of the hexagonal boron layers, which are isoelectronic with graphite if the Mg is oxidized completely to Mg2+. However, comparison of the Mg&z.sbnd;Mg distances in MgB2 with those in several ternary hydrides (Mg2RuH4, Mg3RuH3, and Mg4IrH5), assumed to contain subvalent magnesium as Mg22+, suggests similar subvalent magnesium in MgB2. This incomplete oxidation of Mg to Mg2+ generates the holes in the valence band of the hexagonal boron layers leading to the experimentally observed hole superconductivity in MgB2. [Copyright &y& Elsevier]

Reactions of acetylene, thiophene, or even tellurophene with iron carbonyls have been found to give the tricarbonylferrole iron tricarbonyl, [η4-C4H4Fe(CO)3]Fe(CO)3, which has been shown by X-ray crystallography to contain a metallacyclic FeC4 ring with an Fe–Fe bonding distance of ∼2.5Å to the exocyclic iron atom. This structure has been shown by density functional theory to be the lowest energy C4H4Fe2(CO)6 structure by more than 30kcal/mol relative to the lowest energy isomeric triplet structure. Dissociation of carbonyl groups from this structure requires ∼40kcal/mol per carbonyl group. The central C4H4Fe2 unit in the resulting unsaturated C4H4Fe2(CO) n (n =5, 4, 3) structures is relatively rigid upon carbonyl loss with the Fe–Fe distance decreasing only to ∼2.4Å in even the highly unsaturated C4H4Fe2(CO)3. Thus 16-electron metal configurations with iron–iron single or double bonds rather than the normally favored 18-electron metal configurations are preferred over structures with higher order iron–iron multiple bonds. For the carbonyl-rich species the lowest energy C4H4Fe2(CO)8 structure has one C C double bond of a tetracarbonylferrole ring bonded to an exocyclic Fe(CO)4 moiety with a long Fe⋯Fe non-bonding distance of ∼3.8Å. The low-energy structures for C4H4Fe2(CO)7 either have a dihapto ferrole ligand with an iron–iron bond or a tetrahapto ferrole ligand without an iron–iron bond. However, C4H4Fe2(CO)7 does not appear to be viable since carbonyl dissociation from the lowest energy C4H4Fe2(CO)7 structure to give C4H4Fe2(CO)6 is predicted to be an exothermic process. [ABSTRACT FROM AUTHOR]

Abstract: The trimethylenemethane ligand is found in the experimentally known and structurally characterized mononuclear iron carbonyl derivative (CH2)3CFe(CO)3, which may be regarded as a substitution product of Fe(CO)5. The structures of trinuclear trimethylenemethane iron carbonyl derivatives similarly related to the well-known doubly bridged Fe3(CO)12 structure have now been investigated by density functional theory. The lowest energy [(CH2)3C]Fe3(CO)10 structures likewise have two CO bridges and can be derived from the known Fe3(CO)12 structure by replacement of two terminal CO groups with either an unsymmetrical edge-bridging or face-bridging trimethylene methane ligand while preserving the central Fe3 triangle. However, replacing four CO groups in Fe3(CO)12 with two trimethylenemethane ligands to give [(CH2)3C]2Fe3(CO)8 is predicted unexpectedly to give a very different type of structure. In this structure one edge of the Fe3 triangle in Fe3(CO)12 is broken to form a bent Fe–Fe–Fe chain with the trimethylenemethane ligands bonded to the end iron atoms of this triiron chain. This [(CH2)3C]2Fe3(CO)8 structure appears to be very favorable, since it lies ∼22kcal/mol below alternative structures. We anticipate that both [(CH2)3C]Fe3(CO)10 and [(CH2)3C]2Fe3(CO)8 should be sufficiently stable to be synthesized from the known (CH2)3CFe(CO)3 by thermolysis or photolysis with suitable iron carbonyls. [Copyright &y& Elsevier]

Abstract: Theoretical studies on the known trinuclear cobalt carbonyl derivatives ECo3(CO)9 (E=CH, CF, P, As) predict structures with carbonyl groups bridging each edge of the Co3 triangle in contrast with experiment where structures with all terminal carbonyl groups are found in all cases. However, the energy differences are predicted to be rather small ranging from 4±2kcal/mol for FCCo3(CO)9 to 10±3kcal/mol for AsCo3(CO)9. The global minima for the unsaturated ECo3(CO) n (n = 8, 7, 6) derivatives generally have two (for n =8) or three (for n =7 and 6) carbonyl groups bridging the edges of the Co3 triangle. However, structures with all terminal carbonyl groups are also found in all cases as well as higher energy structures in which one of the carbonyl groups bridges all three cobalt atoms. The fluoromethinyl derivatives FCCo3(CO) n (n =9, 8, 7) are anomalous since their unbridged structures or structures with a carbonyl group bridging all three cobalt atoms are closer in energy to the doubly or triply bridged global minima than is the case for the other ECo3(CO) n derivatives. [Copyright &y& Elsevier]